Gas turbine engines generally have a fuel pump that transfers fuel from a fuel supply to the engine at a desired pressure and flow rate. The engine typically drives the fuel pump through a speed-reducing gearbox that couples the engine to the pump. Since the rotational speed of the pump, and therefore flow rate, is proportional to the rotational speed of the engine, it is common to use a servo-control valve to control fuel flow and a pump relief valve or pressure-regulating valve to control fuel pressure.
The gearbox driven fuel pump has several disadvantages. One is that the size of the pump has to be sufficient to provide a large quantity of fuel to the engine during start-up of the engine, wherein the light-off window is only in the range of 5 to 15 percent of normal engine operational rotational speed. The pump is therefore oversize at operational rotational speed, so that it is three or four times larger than it need be for supplying fuel at operational rotational speed, thereby contributing to excess cost, size and weight of the fuel system. Because the pump then develops excess flow at operational speed, the fuel recirculates through the pump by way of the pump relief valve, thereby contributing to unwanted heating of the fuel.